Melting point is the temperature at which a solid turns into a liquid. Boiling point is the temperature at which a liquid turns into a gas. For water, these are 0 °C (32 °F) and 100 °C (212 °F) at standard atmospheric pressure. Both are phase transitions, but they involve different amounts of energy, respond differently to pressure changes, and happen through distinct physical mechanisms.
What Happens at the Melting Point
When you heat a solid, its molecules vibrate faster and faster. At some point they have so much energy that they can no longer stay locked in the rigid, orderly arrangement of a solid. The substance begins to melt. While melting is underway, something counterintuitive happens: the temperature stops rising. All the heat you add goes into breaking the bonds that hold molecules in their fixed positions rather than making them move faster. Only after every bit of solid has become liquid does the temperature start climbing again.
The energy required to push a substance through this transition is called the heat of fusion. For water, it takes about 334 joules to melt a single gram of ice. That sounds like a lot, but it’s modest compared to what boiling demands, which is a key difference between the two transitions.
What Happens at the Boiling Point
As you keep heating a liquid, its molecules gain enough kinetic energy that some escape from the surface as vapor. The warmer the liquid gets, the more molecules escape, and the vapor pressure rises. Boiling begins at the specific temperature where the vapor pressure inside the liquid equals the pressure of the atmosphere pushing down on it. At that point, bubbles of gas form throughout the body of the liquid, not just at the surface. Just like melting, the temperature plateaus during boiling: every bit of added heat goes into converting liquid to gas until none remains.
The energy cost here is dramatically higher. Turning one gram of liquid water into steam requires about 2,260 joules, roughly 6.8 times more energy than melting the same gram of ice. This ratio holds across many substances. Aluminum, for example, needs 321 J/g to melt but 11,400 J/g to vaporize. The reason is that boiling demands a near-complete separation of molecules from one another, while melting only loosens them from a fixed grid into a fluid that still holds together.
Why Boiling Points Change With Altitude
One of the most practical differences between these two transitions is how they respond to pressure. Boiling point is directly tied to atmospheric pressure: lower the pressure and the boiling point drops. At high elevations, where the air is thinner, water boils below 100 °C. In Denver (about 1,600 meters above sea level), water boils near 95 °C, which is why cooking times increase at altitude.
Melting points, by contrast, barely budge with ordinary pressure changes. The solid-to-liquid transition involves only a small change in volume, so atmospheric pressure has little leverage over it. Ice melts at essentially 0 °C whether you’re at sea level or on a mountaintop. This makes melting point a more stable, reliable reference number for identifying a substance, which is why chemists routinely use it to confirm the identity of a crystalline compound.
Energy Comparison Across Common Substances
The gap between melting energy and boiling energy isn’t unique to water. It shows up consistently because the two transitions ask molecules to do fundamentally different things.
- Iron: 209 J/g to melt, 6,340 J/g to vaporize
- Copper: 207 J/g to melt, 5,069 J/g to vaporize
- Gold: 67 J/g to melt, 1,578 J/g to vaporize
- Mercury: 11.6 J/g to melt, 295 J/g to vaporize
- Methanol: 98.8 J/g to melt, 1,100 J/g to vaporize
In every case, vaporization costs at least several times more energy than fusion. Metals tend to have especially large ratios because their atoms are bound tightly by metallic bonds that require enormous energy to fully overcome during vaporization.
How These Differences Are Used in Industry
The fact that different substances have different boiling points is the basis of one of the most important industrial processes on Earth: fractional distillation. Petroleum refining relies on it entirely. Crude oil is a mixture of hydrocarbons, each with its own boiling point. When crude oil is heated inside a distillation column, lighter components like gasoline and propane vaporize at lower temperatures and rise to the top, while heavier ones like kerosene, lubricating oil, and asphalt remain liquid longer and collect at lower levels.
The same principle separates alcoholic spirits from fermented liquids. Ethanol boils at a lower temperature than water, so heating a fermented mixture drives off alcohol vapor first. That vapor is then cooled and condensed back into liquid, producing a more concentrated spirit. When boiling points of two substances are very close together, a technique called fractional distillation repeatedly condenses and re-vaporizes the mixture inside an insulated column to achieve a cleaner separation.
Melting points serve a different practical role. Because each pure crystalline substance has a sharp, characteristic melting point, chemists use melting point measurements as a quick purity test. An impure sample melts over a wider range and at a lower temperature than the pure substance, making contamination easy to detect without expensive equipment.
The Point Where the Difference Disappears
At extreme temperatures and pressures, the distinction between liquid and gas actually vanishes. Every substance has a critical point: a specific combination of temperature and pressure beyond which liquid and gas merge into a single state called a supercritical fluid. As conditions approach the critical point, the visible boundary between liquid and gas (the meniscus you see in a half-filled container) fades and disappears. Beyond it, there is no boiling point because there is no separate liquid phase to boil.
For water, the critical point sits at about 374 °C and 218 atmospheres of pressure. Under those conditions, water exists as a supercritical fluid with properties of both a liquid and a gas. This isn’t something you encounter in everyday life, but it matters in power plants, certain industrial cleaning processes, and deep geological systems where pressures and temperatures reach those extremes.